Well the possibility jotheonah being SK was definitely present in my mind. Claiming a townie power role (especially one that will look the same as SK--at first, anyway) proved to be really good cover for the first day, and definitely accomplished the goal of surviving. He also was very careful to have his reasoning backing up his choice of target no matter what the target ended up being. Also it seems like with role claiming one-shot vig gives him the opportunity to kill the first night, then lay low the next night or two before killing again.
Axxle's roleclaim, along with Insomniac's confirmation definitely points to jotheonah being SK, but I think I want even a bit more clarification; Insomniac mentioned a bit of looking on mafiascum will find the answer for the earlier question, but I haven't found this answer yet: For role-investigative purposes, does Serial Killer count as a faction or a role? Intuitively to me it seems like a Role Cop investigating Serial Killer should get Serial Killer as the result, unless the Serial Killer is considered something like the one normal member of the Serial Killer faction. [pre-post edit] Just glanced back at Axxle's post, mentioning that SK in this game could be a variant with protection from role checks--definitely possible with this closed game format. Guess that makes my question less useful than I thought it was.
I'm inclined to believe Axxle--probably because I had been considering the possibility that jotheonah was SK, combined with the fact that a fake claim from a mafia Axxle doesn't really make sense to me. OTOH I do have to agree with jotheonah's argument that trying to get him lynched first seems like a better play than roleclaiming right at the start of the day...but then, with mafia and a serial killer, 2 townies can get killed off every night, so if we have a way to knock that number down to 1, doing so is pretty urgent, maybe urgent enough to get it out there right away.
So, leaning towards voting jotheonah, but like Volt mentioned I think it's a good idea to hear from everyone before voting.